In my experience, when the coolant is low you won't feel much heat. It doesn't take much. My son was complaining, "heat doesn't work" in my 1985 300D when he came home last week. I had told him to check the coolant when he left a month ago since I had drawn it down to check the upper hose and knew it would drop a bit as air worked its way out, but he doesn't listen. I only needed to add ~1 cup to hit the full mark (using Evans Waterless) and the heat was much hotter then.

I have also seen the temperature read too high when there is an air pocket. I have used a HF IR gun to measure next to the temperature sensor in the head (goes to dash gage) and the reading agrees quite close w/ the gage. You can get one on sale for <$30 and they prove very useful.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans